Fluoroscopy is a process for obtaining continuous, real-time images of an interior area of a patient via the application and detection of penetrating x-rays. Put simply, x-rays are transmitted through the patient and converted into visible spectrum light by some sort of conversion mechanism (e.g., x-ray-to-light conversion screen and/or x-ray image intensifier). Subsequently, the visible light is captured by a video camera system (or similar device) and displayed on a monitor for use by a medical professional. More recently, a solid-state pixelized flat panel is used for this purpose. Typically, this is done to examine some sort of ongoing biological process in the human body, e.g., the functioning of the lower digestive tract or heart.
Currently, most fluoroscopy is done using x-ray image intensifiers. These are large, vacuum tube devices (i.e., akin to a CRT or conventional television) that typically receive the x-rays in an input end, convert the x-rays to light and then electron beams, guide, accelerate, and amplify the electron beams via internal electrodes, and convert the electron beams to a minified visible image at the device's output end. An example of an x-ray image intensifier is shown in U.S. Pat. No. 5,773,923 to Tamagawa (see FIGS. 1 and 2 and accompanying description).
In designing x-ray support apparatuses, the x-ray device should ideally be positionable for use anywhere around the periphery of a patient in three dimensions (i.e., the X-, Y-, Z-planes). More specifically, it is typically desirable to utilize spherical angulation, where x-rays can be directed from any loci on an imaginary sphere centered on the patient to an isocenter of the x-ray device. (The isocenter is the point of intersection of an axis defined by the x-ray source and receptor and the axis of angulation, i.e., the axis of device rotation.) Other factors to take into account include: maintaining the x-ray beam normal to the x-ray receptor; the size of the examination room, and the room's ability to accommodate large devices; unrestricted access to the patient, especially around the head area; minimizing control complexity and/or the need for computer image correction or manipulation; and, as always, cost.
Most current x-ray device support apparatuses utilize either a parallelogram-shaped construct or a combination of C-, U-, and/or L-shaped arms for x-ray device positioning and (ideally) spherical angulation. An example of the former is shown in U.S. Pat. No. 3,892,967 to Grady et al. (“Grady '967”). In Grady '967, an x-ray source 23 and receptor 22 are positioned with respect to a patient P by way of an angularly-adjustable, pivoting, rotating parallelogram 3, 5, 8, 9. This achieves 360 degrees rotation coverage about the patient P, by virtue of the parallelogram being rotatable about shaft 2, and 55 degrees of head/foot tilt (the arms 8, 9 can be moved in and out.) Thus, the device basically moves in an unrestricted way on the surface of a sphere about the patient, and the x-ray image itself inherently always remains “upright” irrespective of the compound angles used. However, to cover from head to foot on a six foot tall patient, the “throat depth” (clearance) of the support apparatus has to be over six feet. This makes the support apparatus at least ten to twelve feet long, plus the patient tabletop has to travel at least six feet, which means it must be eight to nine feet long. Thus, the entire system is almost twenty feet long, necessitating a twenty-eight or thirty foot long room, which might cause architectural problems.
Because parallelogram-based devices are so bulky, various C-arm based devices have been developed over the years. However, large C-arms are difficult to balance (e.g., a parallelogram can be an entirely mechanically-balanced device), since the entire mass of the C-shaped structure is offset to one side. Accordingly, these have primarily taken the form of a simple, light, balanced, C-shaped arm which holds the x-ray source at one end and the receptor at the other end. The C-shaped arm slides in a journal, and is positionable by way of one or more pivoting arms attached to the journal. Such devices can deliver most of the angular coverage of a parallelogram in a smaller space, but typically have several severe, inherent problems, such as the inability to carry heavy equipment without dangerous power-driven operation, and if the C-arm slides in the journal, the image will rotate due to an interaction of the two axes. Some parts of the spherical view become inaccessible.
With existing C-arm based devices, as the axis of the x-ray beam approaches the horizontal, rotating the horizontal axis only serves to rotate the image, without changing the viewing angle. This results in zero image rotation with a vertical beam, and 100 percent rotation (only) at a horizontal beam. In between 0 degrees and 90 degrees the x-ray beam/positioner angular relationship is complex, and the two rotation axes interact. The result is a tilted image as viewed on the x-ray image screen. This effect can be compensated for by either mechanically rotating the x-ray receptor (and also the source collimator if a square x-ray field is utilized) according to a pre-programmed code, or by implementing an “image de-rotation” scheme where the image, as stored electronically, is manipulated by digital means. However, such systems are expensive, and can ultimately degrade the image. Some angulations critical to cardiology are no longer achievable once the arm is slid in its sliding bearing (i.e., its outer sleeve).
There have been numerous variations in the design and construction of C-arm based x-ray gantries, but two main divisions are apparent: types where the horizontal C-arm axle comes at the patient from the left side, and types where the C-arm axle comes over the patient's head.
For an axle or C-arm mount that is horizontal, approaching the patient from the left side is the ideal orientation, as the head end of the patient is then accessible for anesthesia, nursing care, etc. The tilt range of the “sliding C”, and hence the efficacy of the equipment, in that arrangement is unfortunately reduced as the x-ray beam approaches horizontal, as mentioned above.
The easiest way to visualize this is to note that when the x-ray beam is vertical (“upwards”), any rotation of the generally horizontal main axle of the C-arm mount also predictably tilts the x-ray beam angle a like amount. For example, a 30-degree rotation of the main axle tilts the x-ray beam angle 30 degrees. As long as the main axle or pivot makes close to a right angle (90 degrees) with the x-ray beam, the overall goal is achieved.
However, once the C-arm (a.k.a. the “C”) is “slid” in its mount sleeve, the difficulty appears. To go to the extreme, if the C is slid by 90 degrees from its vertical position, where the beam was vertical, to a new horizontal position with the beam is also horizontal, and then the main pivot is rotated by the same 30 degrees, there is no change in x-ray beam angle at all. The 30 degree rotation of the main axle only serves to rotate the image receptor, which rotates the displayed image.
Two situations result with the C slid to the horizontal beam position:
1) no head-to-foot, or cranio-caudal, angulation is possible; and
2) the image rotates on the viewing screen if the axle is turned.
For intermediate positions along the sliding C motion, a mix of the two situations appears, and they interact in a complex way. The essence of the situation is that for x-ray beams near lateral, with the C on the left side of the patient, there is very little cranio caudal angulation available and image rotation is severe.
These problems led to the “parallelogram” support and positioning apparatus for x-ray equipment disclosed in the Grady '967 patent, mentioned above, issued in 1975 to John K. Grady (the present Applicant) and David B. Rice. That “parallelogram” apparatus has none of the above issues, but obstructs the head of the patient and requires a long room. The image on a parallelogram does not rotate at all for all spherical views.
A first prior solution for the C-arm construction is to move the C-arm to the head end permanently. However, that obstructs the head end. It also gives very limited coverage of the patient, as the C-arm radius is limited by the floor and the center point must be inside the patient. Consequently, the patient's head will strike the C-arm interior diameter before the beam is in the pelvic area.
A second prior art solution is to mount the C-arm on a third axis pivot, which allows it to be moved from the left side to the head end of the patient, depending on clinical requirements. There are many existing variations along this theme, with floor and ceiling pivots, dual third axis pivots generating on offset, etc. All suffer from the complexities of a third pivot, coordinating that pivot to the other two, and adding a new source of image rotation that interacts with the other two in a complex way. With the recent advent of software based 3-D image reconstructions, the impact of image rotation or shifts is really severe, often becoming a limiting factor in the data integrity.
These problems can be solved for a stationary catheterization (“cath”) lab with a different approach—see U.S. Pat. No. 6,789,941 (“Grady '941”), issued Sep. 14, 2004 to John K. Grady. Grady '941 describes a C-shaped arm with a pivot fixed at 90 degrees to the beam, the whole thing traveling inside an arc. Note that there is no sliding mount or sleeve on the C-arm mount at all; that deletion eliminates image rotation. The “axle pivot” (equivalent) is actually moving with the C-arm, always at 90 degrees to the main beam, a critical design differentiator.